This invention relates to stored program controlled apparatus and, in particular, to apparatus with a capability for remote updating of its entire set of programs.
Stored program controlled apparatus can conveniently be divided into two types; one where the stored programs are completely unalterable under normal operating circumstances, and the other where the stored programs are alterable, at least at times, during normal operation. In apparatus of the first type, the program is often executed automatically and the user does not even know that the controlled apparatus is "stored program controlled". This is typically the case in equipment that is designed for people who are not knowledgeable in computers and for whom the equipment is just a tool of the trade. "Point of sale" terminals, such as check-out terminals at a supermarket, are a good example. Modems are another example. People who use this equipment desire fail-safe operation and they do not want to be bothered with loading programs, fixing program bugs, installing updated versions of software, etc.
One approach to programming such equipment is to imprint the program into read-only-memory integrated circuits and physically install the circuits into the equipment. The problem with this approach is that updated versions of the program require the creation of new sets of read-only memories and new installations.
When a communication link is present, "downloading" the programs to the equipment from a remote processor, through the communication link, forms another approach for programming the equipment. It has been known in the art for some time that it is feasible to download limited types of control information from a remote processor. It is also known to download entire machine language application programs. Often such equipment does not include writable non-volatile store, such as a hard disk, so the programs are stored in battery protected read/write memories. This is an unattractive solution because it leaves a substantial portion of program memory to be at risk. To mitigate this problem, U.S. Pat. No. 4,724,521, suggests storing within read-only memories of the local equipment a number of general purpose routines which comprise instructions to be executed by the central processing unit to accomplish a particular program task. These, in effect, form a set of macro-instructions. The downloaded machine language program utilizes these macro-instructions to the extent possible, and thereby achieves flexibility without the need to download substantial amounts of program code.
In all of the known approaches, however, there is a program portion in the local equipment that is resident in a read-only memory, and its contents are not changed. That resident portion contains "boot-up" segments and program segments that are necessary to maintain the communication between the remote processor and the local equipment (so that the process of downloading the programs can continue). This set of programs is the "essential programs" (EP) set. This set of programs should, of course, be a non-volatile store because there is always a possibility of power loss.
The fact that the EP set is needed to maintain communications presents a problem when the EP set itself needs to be modified or updated. Indeed, that is often the case with modems, where essentially the sole function of the modem software is to support communication.